The aim of the research program proposed here is to explore how established drugs influence the permeability of capillary endothelium, peritoneal mesothelium, and eventually other non-stratified epithelia (e.g., amnion), in order to learn how these influences may help us to understand: (a) biological mechanisms of solute transfer across epithelial barriers, with special emphasis on diffusion; (b) the process of water transfer across epithelial barriers, with particular emphasis on filtration under hydrostatic pressure gradients; (c) humoral and neural mechanisms that may control and regulate epithelial permeability in health and disease; (d) mechanisms of drug action at cellular and subcellular (receptor) levels. Transcapillary transport is the focus of current studies, particularly in skeletal and heart muscle in anesthetized cats. Rates of tissue clearance of test substances injected into muscle make it possible to detect and assess drug effects on smooth muscle in arterioles and precapillary sphincters, on capillary blood flow, on capillary diffusion capacity and eventually on the permeability of the capillary wall. The "drugs" under consideration include angiotensin, prostaglandins, neurotransmitters and common synthetic vasoactive drugs.